Conventional hydraulic clutch release mechanisms generally include a hydraulic fluid reservoir connected to a master cylinder. Hydraulic fluid is forced from the master cylinder, as clutch pedal is depressed, into an expandable fluid chamber in a slave cylinder. Expansion of the fluid chamber moves a piston out of the slave cylinder, which is fixed relative to a transmission casing. The moving piston compresses the fingers of a diaphragm-type clutch release spring to disengage the clutch. Generally, the spring fingers are in the form of levers that give a mechanical advantage to the motion of the piston. The position of the spring fingers, relative to the transmission casing when the clutch is engaged, is determined by how much wear has occurred to the friction disk of the clutch. Because of the mechanical advantage of the spring fingers, clutch wear may translate into a significant progressive shifting of the clutch engaged position of the fingers throughout the life of the clutch. The part of the mechanism that directly engages the spring fingers is generally a bearing mounted to the front of the piston. The bearing should be maintained in a continual light engagement with the spring fingers when they are in the clutch engaged position. This assures that the piston will be ready to compress the spring fingers with no lost motion when the clutch pedal is depressed. Since the slave cylinder is fixed relative to the transmission, the position of the piston in the slave cylinder and, therefore, the volume of hydraulic fluid in the fluid chamber, will have to vary as the clutch wears throughout its life. Since the spring fingers generally are designed to move closer to the transmission casing with clutch wear, a provision is usually made for extra volume in the slave cylinder at the beginning of clutch life. Then, fluid is allowed to return to the reservoir as the clutch wears. Providing for a constant volume of hydraulic fluid within the mechanism independent of clutch wear would obviate this design consideration.
Conventional hydraulic release mechanisms also require moving seals between the piston and the slave cylinder to prevent leakage of hydraulic fluid, and these seals are subject to wear with time. One solution is to form the expandable fluid chamber with a flexible bellows. The fluid chamber then expands as the bellows expands, with no moving seal. Examples of hydraulic release mechanisms of may be found in the U.S. patent to Garrett et al, U.S. Pat. No. 4,51,937 and in European Patent EP No. 0 074 6781 A1. In the mechanisms there disclosed, the expansion of the fluid chamber formed by the bellows pushes a piston, or similar release member, toward the spring fingers. The bellows forms convolutions about the piston, which roll out as the moving piston compresses the spring fingers. However, release mechanisms of the bellows type are also subject to the change in volume caused by the clutch wear discussed above, because the bellows is generally fixed relative to the transmission. This may present special problems in this type of mechanism, beyond the necessity of providing for extra volume in the fluid chamber. Providing such extra volume entails larger bellows sizes and longer convolution lengths therewithin. This may cause extra strain in the rolling convolutions, and the potential for stretching of the bellows material and lost work increases with bellows size. A constant volume could be even more beneficial, then, with the bellows type of hydraulic release mechanism.